In the mid-1930s, the American cartoonist and humorist James Thurber and his wife made a trip to Loch Ness, partly in the hope of seeing its famous monster or speaking to those who had. It was only a couple of years since Nessie fever had been sparked, in 1933, by a local hotel owner and his wife, who reported seeing a whale-like creature that left a froth of water in its wake when it disappeared beneath the waters.

Thurber - who was partially blind – might not have been the best person to add to the official record of sightings, but he was certainly a man who understood the big questions of the day. One of his essays is titled "Is Sex Necessary?" Now, here he was, asking if Nessie was real.

For a man whose work had previously suggested no subject was too serious to be lampooned, his New Yorker article about that trip and a return visit 20 years later, called "There’s Something Out There" was unusually sober. It seems that Thurber could laugh at almost anything except the notion that beneath the steely waves of Loch Ness might lurk a fantastical creature which had been in existence since the 6th century, when St Columba banished a “water-beast” to the deeps, thereby saving the life of a Pict it had been about to devour.

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If the monster had been alive in Columba’s day, by the time the Thurbers took their holiday it would surely have been growing too weary to poke its head above water. Of course, if Nessie truly is some form of whale, she might feasibly have a lifespan of one or two hundred years, suggesting the loch has been the breeding ground for umpteen Nessies down the centuries.

Nowhere do reality and dreams collide more sharply than Loch Ness. In one camp are the believers, their enthusiasm undaunted despite the fact that numerous attempts to prove the beast’s existence have failed to find any evidence. Last century various expeditions were launched, including a sonar sweep in 1987 that produced nothing. Last month, to mark the 90th anniversary of the 1933 sighting, a two-day hunt was conducted with around 100 volunteers. It almost produced a result, as the team’s leader, Alan McKenna, explained: “We did hear something. We heard four distinctive ‘gloops’. We all got a bit excited, ran to go make sure the recorder was on, and it wasn’t plugged in.”

In the other camp are the hard-headed rationalists - scoffmonsters, as Thurber dismissively called them - who think Nessie’s existence is all in the mind, a matter of misidentification, collective delusion or wishful thinking. The undulating humped 45ft beast sporadically spotted breaching the water’s surface is explained by them as either a giant squid, seal or eel, or a “wake-monster”, ie rippling waves. Waves seem the likeliest explanation to me, but the belief that many Nessie enthusiasts cling to, which keeps them camped out on its banks, is that the loch is home to a Jurassic-age reptile, which accounts for the 1,000-plus official sightings recorded.

What is interesting is why lack of evidence does nothing to dent believers’ trust. One in particular will allow of no other possibility than that Nessie is simply avoiding attention. Steve Feltham, who first visited Loch Ness as a seven-year-old, packed in his job in Dorset 32 years ago and headed north. Since then he has been living in a van-cum-cabin on the shores of the loch with his binoculars, gaining himself a place in the Guinness Book of Records for the longest continuous monster-hunting vigil of Loch Ness. What is it that Nessie represents, and why does her existence matter so much?

For an atheist or agnostic, the same question could be put to those with religious faith. In this instance, though, Nessie taps into something more tangible: hopes of finding a relic of our primeval past which, if discovered hale and hearty, would keep the thread between us and unimaginably distant history unbroken. As well as affirming our connection with everything that has gone before us, it would put into perspective our dangerously human-centric view of the planet. I can see the attraction in that.

Nor is it technically impossible that a creature that was alive when fossils were being laid down is still among us. As Thurber points out, it had been assumed that the coelacanth had become extinct 66 million years ago until, in 1938, one was found in South Africa. In other words, experts can be confounded, which is another of Nessie’s attractions. Her existence, if proved, would wrong-foot our super-scientific and technology-driven world, and instead offer a glimpse of a realm we can all understand and relate to.

At the same time, Nessie is symbolic of an ancient Scotland, a place filled with legends such as kelpies, selkies and wulvers. She (or he?) also reflects atavistic fears about dark, deep waters, and what might lurk beneath them.

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Like the Yeti, she has become not just a tourist attraction but a national icon; unlike the Yeti, her lair is accessible to anyone with a car or bus pass. As Thurber wrote, she is “reputed to have brought more income to Scotland than any other single attraction except Scottish whiskey”. Hence the newly revamped Loch Ness visitor centre, opened this summer.

Her allure for tourists no doubt explains why, in 1940, Goebbels ran a two-page spread in the Hamburger Illustrierte debunking her as a myth invented by hoteliers to attract visitors. In the same year, Mussolini’s paper, Popolo d’Italia, took a different tack, announcing that because of increased bombing raids on Britain, she had been killed by a direct hit. Either way, the fascisti understood Nessie’s status in the country’s affections, and were hell-bent on denting British morale.

It is almost impossible to prove a negative, which explains why belief in her persists. Her elusiveness is attributed to wiliness and fear of humankind, rather than confirmation that the loch is filled only with pond life. I fall into the category of scoffmonster, but perhaps it’s not so odd that someone like Thurber had faith. One of his most famous cartoons neatly encapsulates the Nessie debate. A couple are in bed, unaware there is a seal behind them on the headboard: “All right, have it your way,” says the wife, “you heard a seal bark.”